The British Wind Energy Association (BWEA) has agreed to scale down its calculation for the amount of harmful carbon dioxide emission that can be eliminated by using wind turbines to generate electricity instead of burning fossil fuels such as coal or gas.

The move is a serious setback for the advocates of wind power, as it will be regarded as a concession that twice as many wind turbines as previously calculated will be needed to provide the same degree of reduction in Britain's carbon emissions.

A wind farm industry source admitted: "It's not ideal for us. It's the result of pressure by the anti-wind farm lobby."

For several years the BWEA – which lobbies on behalf of wind power firms – claimed that electricity from wind turbines 'displaces' 860 grams of carbon dioxide emission for every kilowatt hour of electricity generated.

However it has now halved that figure to 430 grams, following discussions with the Advertising Standards Authority (ASA).

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Hundreds of wind farms are being planned across the country, adding to the 198 onshore and offshore farms - a total of 2,389 turbines - already in operation. Another 40 farms are currently under construction.

Experts have previously calculated that to help achieve the Government's aim of saving around 200 million tons of CO2 emissions by 2020 - through generating 15 per cent of the country's electricity from wind power - would require 50,000 wind turbines.

But the new figure for carbon displacement means that twice as many turbines would now be needed to save the same amount of CO2 emissions.

While their advocates regard wind farms as a key part of Britain's fight against climate change, opponents argue they blight the landscape at great financial cost while bringing little environmental benefit.

Dr Mike Hall, an anti-wind farm campaigner from the Friends of Eden, Lakeland and Lunesdale Scenery group in the Lake District, said: "Every wind farm application says it will lead to a big saving in the amount of carbon dioxide produced. This has been greatly exaggerated and the reduction in the carbon displacement figure is a significant admission of this.

"As we get cleaner power stations on line, the figure will get even lower. It further backs the argument that wind farms are one of the most inefficient and expensive ways of lowering carbon emissions."

Because wind farms burn no fuel, they emit no carbon dioxide during regular running. The revised calculation for the amount of carbon emission they save has come about because the BWEA's earlier figure did not take account of recent improvements to the technology used in conventional, fossil-fuel-burning power stations.

The figure of 860 grams dates back to the days of old-style coal-fired power stations. However, since the early 1990s, many of the dirty coal-fired stations have been replaced by cleaner-burning stations, with a consequent reduction in what the industry calls the "grid average mix" figure for carbon dioxide displacement.

As a result, a modern 100MW coal or gas power station is now calculated to produce half as many tonnes of carbon dioxide as its predecessor would have done.

The BWEA's move follows a number of rulings by the ASA against claims made by individual wind farm promoters about the benefits their schemes would have in reducing carbon emissions.

In one key adjudication, the ASA ruled that a claim by Npower Renewables that a wind farm planned for the southern edge of Exmoor National Park, in Devon, would help prevent the release of 33,000 tonnes of carbon dioxide into the atmosphere was "inaccurate and likely to mislead". This claim was based on the 860-gram figure.

The watchdog concluded: "We told Npower to ensure that future carbon savings claims were based on a more representative and rigorous carbon emissions factor."

The ASA has now recommended that the BWEA and generating companies use the far lower figure of 430 grams.

In a letter to its members, the BWEA's head of onshore, Jan Matthiesen, said: "It was agreed to recommend to all BWEA members to use the single static figure of 430 g CO2/kWh for the time being. The advantage is that it is well accepted and presents little risk as it understates the true figure."

This is now the figure given on the BWEA's website. The organisation will also be forced to lower its claim for the total amount of carbon dioxide emission saved by the 2,389 wind turbines currently operating around Britain.

But the association denied the change weakened the case for wind farms.

Nick Medic, spokesman for the BWEA, said: "Wind farms are still eliminating emissions. The fact is that fossil fuel burning power stations belch out CO2 and wind farms don't. That has not changed.

"The fact is we need to reduce carbon emissions, however you account for them. But there are people who just don't like wind farms and will use any argument against them."